This invention relates to a process for reducing residual monomers, diluents, solvents and other residues, particularly styrene and acrylonitrile monomers in styrenic polymers and more particularly to products such as packaging materials shaped from the melted polymer wherein free styrene and acrylonitrile monomers therein are reduced.
Thermoplastic polymer compositions, particularly those containing polymerized styrene, can be shaped into a wide variety of useful articles by conventional techniques such as extrusion, milling, molding, drawing, blowing, etc. Applications for such shaped articles are widespread and include structural units where properties such as low thermal deformation impact resistance (when a rubber component such as a butadiene polymer or copolymer is present) and high gloss are required. When acrylonitrile is present with styrene in a copolymer, the compositions uniquely exhibit excellent solvent resistance and low permeability to liquids and gases which make them especially useful as a lightweight substitute for glass in packaging and particularly in the manufacture of bottles, film, sheet, tubs, cups, trays and other containers for liquids and solids.
In manufacturing such polymer compositions, it is well known that free, unconverted, styrene monomer remains absorbed within the polymer particles when polymerization is not 100 percent complete and which is therefore present in products formed therefrom. Additionally present are small amounts of other monomers, solvents, oligomers, catalyst or volatile condensation products which may include for example acrylonitrile, ethylbenzene, toluene, etc. These materials may be detrimental to the ultimate product by reason of off-taste, toxicity, or downgrading of polymer properties via plasticization, depolymerization, etc. Also, government regulatory agencies are moving toward establishing maximum permissible levels of various monomers in the environment on grounds that excess levels may constitute a health hazard, and particularly regulations have been applied to packaging materials intended to contact food, beverages, pharmaceuticals, cosmetics and the like for which application of styrenic copolymers are especially suited.
Even though previously employed processes for vacuum stripping of polymer melts or solvent extracting finished polymer, as for example with alcohols or water, does reduce residual monomers, it has been found that free styrene and acrylonitrile monomers are thermally regenerated during subsequent high temperature operations such as melt processing. Depending on the level of monomers present in the polymer before melting, such an increase of regenerated monomer could lead to monomer leaching into foodstuffs which themselves simulate solvents and/or having the monomers released into the atmosphere around melt processing equipment.
Attempted alternatives to stripping the polymers have included for example chemically reducing various monomers with scavenger compounds. One example of such a prior art process may be found in U.S. Pat. No. 4,274,984.
It is undesirable in many food packaging applications to introduce scavenging compounds because they may not be inert to the other constituents of the polymer melt or may come in contact with the packaged food product. Furthermore, the prior art processes have been ineffective in reducing other residuals of the melt such as acrylonitrile and ethylbenzene to acceptable levels. Additionally, the prior art methods are relatively time consuming and fail to improve any property of the polymer other than residue reduction.